Ion exchange system structure with a microtextured surface, method of manufacture, and method of use thereof

ABSTRACT

A method for roughening a surface of an ion exchange system structure using laser interaction with a surface. The laser surface roughening process allows the use of a wide range of substrates such as metals, ceramics, silicates, polymers and the like, including varieties which can not be fabricated in a fine fibrous structure. The surface roughened ion exchange system structure may be used as an ion-exchange media in applications such as fuel cells, batteries, and other catalysis systems where a high surface exchange area is desirable.

TECHNICAL FIELD

[0001] The technical field relates to microtextured surfaces in an ionexchange system structure and a method for making the same. Themicrotextured ion exchange system structure may be used inelectrochemical devices, including fuel cells, batteries, sensors,electrolyzers and the like.

BACKGROUND

[0002] A desirable feature for ion-exchange media used in applicationssuch as fuel cells, batteries, sensors, electrolyzers and othercatalysis systems is the ability to deliver the highest exchange surfacearea while minimizing the size and weight of the entire system. Animportant metric used in comparing the performance of different systemdesigns is the ratio of the exchange area to the volume of the system.For example, in a fuel cell, the increased contact area between theelectrolyte, reactants and the catalytic surface results in an increasein the number of reactions per unit time. Therefore, the development ofmethods to increase surface area is critical to the improvement oftechnologies dependant on ion exchange. Common methods of increasingsurface area fall into one of three categories, namely, microfibers,porous materials and roughened or microtextured surfaces.

[0003] With regard to the last category, a well known method forproducing roughened surface on a nano scale is the plasma process. Theprocess, however, requires high temperature and pressure that may damagecertain substrates. Other methods of roughening include the impingementof sand or other particulates against a surface or the use of abrasivesmounted on substrates; grinding wheels and sandpaper are examples. Theseprocesses, however, only provide limited surface area enhancement andare fraught with problems associated with contamination.

[0004] In catalysis systems, such as fuel cells, batteries, sensors, andelectrolyzers, the ion exchange membrane is typically coated with acontinuous or discontinuous layer of catalyst to promote the rates ofchemical reactions. Commonly used catalysts include platinum (Pt) and Ptalloys, vanadium (V) and V alloys, titanium dioxide, iron, nickel,lithium and gold.

[0005] A fuel cell is an electrochemical apparatus wherein chemicalenergy generated from a combination of a fuel with an oxidant isconverted to electric energy in the presence of a catalyst. The fuel isfed to an anode, which has a negative polarity, and the oxidant is fedto a cathode, which, conversely, has a positive polarity. The twoelectrodes are connected within the fuel cell by an electrolyte totransmit protons from the anode to the cathode. The electrolyte can bean acidic or an alkaline solution, or a solid polymer ion-exchangemembrane characterized by a high ionic conductivity. The solid polymerelectrolyte is often referred to as a proton exchange membrane (PEM).

[0006] The simplest and most common type of fuel cell employs an acidelectrolyte. Hydrogen is ionized at an anode catalyst layer to produceprotons. The protons migrate through the electrolyte from the anode tothe cathode. At a cathode catalyst layer, oxygen reacts with the protonsto form water. The anode and cathode reactions in this type of fuel cellare shown in the following equations:

Anode reaction (fuel side):2H₂ →4H ⁺+4e⁻  (I)

Cathode reaction (air side): O₂+4H⁺+4e⁻→2H₂O  (II)

Net reaction: 2H₂+O₂→2H₂O  (III)

[0007] The goal is complete hydrogen oxidation for maximum energygeneration shown in the equation. However, the oxidation and reductionreactions require catalysts in order to proceed at useful rates.Catalysts are important because the energy efficiency of any fuel cellis determined, in part, by the overpotentials necessary at the fuelcell's anode and cathode. In the absence of an catalyst, a typicalelectrode reaction occurs, if at all, only at very high overpotentials.

[0008] One of the essential requirements of typical fuel cells, andindeed any ion exchange system, is easy access to the electrode and alarge surface area for reaction. This requirement can be satisfied byusing an electrode made of an electrically conductive porous substratethat renders the electrode permeable to fluid reactants and products inthe fuel cell. To increase the surface area for reaction, the catalystcan also be filled into or deposited onto a porous substrate.

[0009] However, these modifications result in a fragile porous electrodethat needs additional mechanical support. An alternative is to sinter aporous coating on a solid substrate and then fill or re-coat the porouscoating with a catalyst. The sintering process, however, is a multiplestep procedure that requires baking at high temperatures.

[0010] In U.S. Pat. No. 6,326,097 to Hockaday, a surface replicatechnique is used to form an “egg-crate” texture on a membrane in amicro-fuel cell. The catalyst and metal electrode are applied to thesurface of the membrane, and then the membrane is etched away so thatthe catalyst and electrode surfaces replicate that texture. Thisprocedure is complicated, requiring blind etching and many separateoperations.

[0011] Others have used silicon micro machining to increase theeffective surface area of an electrode (Lee, S. J. et al., MiniatureFuel Cells with Non-Planar Interface by Microfabrication. In: PowerSources for the New Millenium, Jain, M. et al. (eds.), ProceedingsVolume 2000-22, The Ion exchange Society Proceeding Series, Pennington,N.J., 2000). Etching of silicon is a very time-consuming process.

SUMMARY

[0012] A process using laser interaction with a surface to enhance theproduction of ions at a surface of an ion exchange system structure isdisclosed. In one embodiment, laser radiation is applied to a surface ofan electrode substrate near an ablation threshold of the substrate tocreate a variety of shapes including cone-like and fibrous structures.In another embodiment, the laser radiation is applied to the surface ofan electrode in an ion exchange membrane system to melt, boil or quenchpart of the surface to create a rough and porous layer at the surface.In yet another embodiment, an ion exchange membrane with a roughenedsurface is prepared by solidifying a solution on a laser roughenedsurface or in a mold having a laser roughened inner surface, or bystamping an ion exchange membrane substrate with a laser roughenedsurface.

[0013] The laser radiation can be applied to a surface of an electrodeafter fabrication of the electrode and, therefore, reduces the level ofdamage and/or contamination of the surface. Since the roughness isformed only where the laser beam strikes the surface, the surfaceroughening can be patterned to fit a specific application with verytight positional accuracy. In addition, the laser roughening operationcan be performed quickly in an ambient environment by a batch process oron a continuous web, manufacturable process.

[0014] The laser surface roughening process allows the use of a widerange of electrode substrates such as polymers, ceramics, silicates, andthe like, including varieties which can not be fabricated in a finefibrous structure. Using the laser roughening method, a solid film maybe treated to create enhanced surface areas in a single step as opposedto the multiple-step processing required to fabricate a nonwoven solidcomposite. The surface roughened electrode may be used in an ionexchange system in applications such as fuel cells, batteries, and othercatalysis systems where a high surface exchange area is desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The detailed description will refer to the following drawings, inwhich like numerals refer to like elements, and in which:

[0016]FIGS. 1A, 1B and 1C are schematics illustrating the equipment andprocess of cone formation on a surface by laser radiation.

[0017]FIG. 2 is a schematic of a laser roughened surface where twoscales of roughness are produced.

[0018]FIG. 3 depicts an embodiment of cone formation using particlesimbedded in the substrate.

[0019]FIG. 4 shows an alkaline direct methanol fuel cell with asurface-roughened electrode.

[0020]FIG. 5. shows a direct methanol fuel cell with a surface-roughenedflex as PEM.

[0021]FIGS. 6A, 6B and 6C depict the use of a roughened surface as amold or embossing tool for producing PEM.

DETAILED DESCRIPTION

[0022] The interaction of laser radiation with a material may result insignificant changes to the morphology of the surface and near surface ofthe material. There are a number of mechanisms by which the surfacechange may occur. Examples include: selective ablation by imaging thebeam using contact or projection mask, ablation-induced cone formation,preferential ablation of the matrix of a multi-phase material,preferential etching of grain boundaries, boiling and rapidsolidification of the surface material and other mechanisms. Whenproperly controlled, the three-dimensional surface topography producedby these treatments results in a surface area that is many times greaterthan the original surface. The laser radiation thus provides anothermethod for producing ion exchange membranes with enlarged exchangesurfaces.

[0023] When light is applied to a light absorbing material, it ispossible to change the surface of the material significantly and to forma plethora of different surface structures such as waves, ripples, pits,nodules, cones and cracks. The character of features produced is basedon the mechanisms that create the features. The mechanisms themselvesare varied and depend on the characteristics of the light and the natureof the interaction of the photons and the material.

[0024] In one embodiment, an excimer laser 101 is directed towards asubstrate 105 as shown in FIGS. 1A and 1B. The wavelength, fluence andenergy of a laser beam 103 are chosen such that photons 107 removematerial from surface 109 of the substrate 105 in a process known asablation. During ablation, a plume of ablation debris 111 is ejectedabove the surface 109 of the substrate 105 (See FIG. 1C). By choosingthe correct frequency and feed rate and a fluence that is above thesubstrate ablation threshold and below the debris ablation threshold, itis possible to encourage the resettling of the debris 111 back onto thesurface 109 of the substrate 105. The resettled debris 111 will shadowportions of the underlying substrate material from the laser light andthe substrate 105 will be ablated non-uniformly, forming a conestructure 113 as shown in FIG. 1C. The cone structure 113 with thedebris 111 attached is a useful material for an ion exchange membrane.The laser ablation process creates two size orders of roughness on thesurface of the substrate 105 (FIG. 2). A large-scale roughness (i.e.,the cone structure 113) having a size on the order of 1-100 micron iscreated due to the shadowing provided by the debris 111. A fine scaleroughness in the size range of tens of nanometers (indicated by theexpanded portion of the diagram in FIG. 2) is created due to thedeposited ablation debris 111. This combination of large scale and finescale roughness significantly increases the surface area exposed for ionexchange.

[0025] Redeposition of the debris 111 is just one of many methodscapable of providing shading of the substrate 105 to form structures onthe surface 109. FIG. 3 shows another embodiment where particles 115 ofhigher ablation threshold have been pre-deposited inside the substrate105. The substrate surface 109 is ablated down to expose the particles115, which then shadow the underlying material forming the conestructure 113. In other embodiments not shown, various masks may beinserted between the light source and the substrate 105 or deposited onthe surface of the substrate 105. Examples of masks include contactmasks, projection masks, films, particles and coatings deposited on thesurface and the like. Diffractive optics may be used to project an imageon the surface 109.

[0026] There are other embodiments where the mechanisms are quitedifferent. For example, in metals and glasses, it is possible to meltand even boil the surface of the substrate with a laser thereby forminga rough surface.

[0027] Membrane materials that may be surface treated by laser radiationinclude, but are not limited to, metals, plastics, silicon, ceramics andcomposites there of. Any material that can be manipulated with a laseris a potential candidate. The types of light sources capable of inducingsuch changes on the surface of a material are well known in the art.Examples include gas lasers such as excimer and solid state lasers suchas YAG lasers as well as flash lamps, UV exposure tools and the like.What is important is to match the material with a light source that willinteract with the desired material sufficiently to provide theroughening effect.

[0028] The membrane with laser roughened surface may be used inapplications such as fuel cells, batteries, and other catalysis systemswhere a high surface area to volume ratio is desirable. FIG. 4 shows anembodiment where a laser treated substrate is used in an electrode 131of an alkaline direct methanol fuel cell 200. In this embodiment, thealkaline direct methanol fuel cell 200 contains an anode 131 (fuelelectrode) and a cathode 141 (air electrode), separated byfuel/electrolyte mixture 133. The fuel/electrolyte mixture 133 may bemethanol (fuel) dissolved in a KOH solution (electrolyte). Thefuel/electrolyte mixture 133 is in full contact with both the anode 131and cathode 141. The application of surface roughened material in theanode 131 would amplify the effective surface reaction area and resultin a higher reaction rate.

[0029] The anode 131 may include a plastic substrate 105, such as Kaptonor any other suitable polymer, with a laser textured surface 109 that iscovered with a conductive layer 135 and a catalyst layer 137. Theconductive layer 135 may be formed by depositing onto the texturedsurface 109 a conductive material by electroless plating, sputtering,atomic layer deposition, or any other process that is capable of coatingthe surface of a non-conductive material. The conductive material may beany material of interest such as Ni, Cu, Al, Fe, Zn, In, Ti, Pb, V, Cr,Co, Sn, Au, Sb, Ca, Mo, Rh, Mn, B, Si, Ge, Se, La, Ga, Ir, or an alloy.The catalyst layer 137 may be Pt or Pt alloys such as Pt-Ru andPt-Ru-Osor, V, V alloys, titanium dioxide, iron, nickel, lithium, gold,or any other material of interest. The catalyst layer 137 may bedeposited onto the conductive layer 135 by electroplating, atomic layerdeposition, chemical vapor deposition, sputter deposition or any otherprocess that is capable of coating a conductive surface. The catalystmay be applied so that it forms a discontinuous surface layer 137 overthe conductor layer 135. The formation of a discontinuous catalyst layer137 is facilitated by the cone structure, upon which catalytic materialcan be preferentially applied to the tops of the cones. Alternatively,the non-conductive textured surface 109 may be directly coated with acontinuous catalyst layer 137 (which will serve both conductive andcatalytic functions) by atomic layer deposition, chemical vapordeposition, sputter deposition or any other process that is capable ofcoating a non-conductive surface.

[0030]FIG. 5 depicts another embodiment utilizing the surface roughenedelectrode membrane in a fuel cell with a solid polymer electrolytemembrane (PEM). In this embodiment, a fuel cell 300 contains an anode151 (fuel electrode) and a cathode 153 (air electrode), separated by aPEM 155. The anode 131 is made from a surface roughened flex material157 covered with a conductor layer 135 and a catalyst layer 137. Thesurface roughened flex material 157 is thinned and etched from the backside to form micro-machined pores 139 so that fuel 143 on the anode sidecan reach the active catalytic surfaces 137 through the openings 139.Here again, the surface roughening of the flex material 157 provideshigher reaction rates and more efficient operation.

[0031] In another embodiment, a substrate with laser-roughened surfaceis used as a mold or a stamp to produce a PEM with a roughened surfaceor surfaces. As shown in FIG. 6A, an electrolyte material is melted ormixed with a solvent to form a solution 161. The solution is cast onto alaser roughened surface 109 and allowed to solidify into a membrane 163,which is then separated from the surface 109. In this manner, a surface165 of the membrane 163 is a negative relief of the laser roughenedsurface 109 (FIG. 6B). The membrane 163 then may be covered with aconductor 135 and a catalyst 137 and may be used as a PEM for a fuelcell.

[0032] The electrolyte material includes, but is not limited to,sulfonated, phosphonated, or carboxylated ion-conducting aromaticpolymer and perfluorinated co-polymer. The solvent includes, but is notlimited to, lower aliphatic alcohols such as propanol, butanol, andmethanol and water or a mixture thereof FIG. 6C depicts anotherembodiment wherein an ion exchange membrane 167 with a textured surfaceis produced by stamping the membrane 163 and a laser roughened surface109 with a roller 171.

[0033] In yet another embodiment, the solution 161 may be poured into amold having laser roughened inner surfaces to form an ion exchangemembrane 163 with textured surfaces on both the up-side and lower sideof the membrane.

[0034] The ion exchange membrane with textured surfaces on both sidesmay be used as a PEM in a PEM-electrode structure, wherein both sides ofthe PEM are covered by conductor layers and catalyst layers. Porouselectrodes that allow fuel delivery and oxygen exchange can then bepressed against the catalyst layers of the PEM to form the PEM-electrodestructure.

[0035] Although embodiments and their advantages have been described indetail, various changes, substitutions and alterations can be madeherein without departing from the spirit and scope of the laserroughening process and the use of surface roughened products as definedby the appended claims and their equivalents.

What is claimed is:
 1. A substrate for an ion-exchange system structure,said substrate comprising a surface wherein at least a portion of thesurface is irradiated by a laser radiation to enlarge a reactive surfacearea.
 2. The substrate of claim 1, wherein the portion of the surface isirradiated by exposing the surface to the laser radiation near anablation threshold of the membrane.
 3. The substrate of claim 1, whereinthe portion of the surface is irradiated by melting, boiling, orquenching part of the surface with laser radiation.
 4. The substrate ofclaim 1, wherein the laser irradiated surface is coated with a layer ofconductive material.
 5. The substrate of claim 4, wherein the conductivematerial is a metal or an alloy.
 6. The substrate of claim 4, whereinthe layer of conductive material is further coated with a continuous ordiscontinuous layer of catalytic material.
 7. The substrate of claim 6,wherein the catalytic material is selected from a group consisting ofPt, Pt alloys, V, V alloys, titanium dioxide, iron, nickel, lithium andgold.
 8. The substrate of claim 1, wherein the laser irradiated surfaceis coated with a continuous or discontinuous layer of catalyticmaterial. 9 The substrate of claim 8, wherein the catalytic material isselected from a group consisting of Pt, Pt alloys, V, V alloys, titaniumdioxide, iron, nickel, lithium and gold.
 10. The substrate of claim 8,further comprising micro openings wherein a fuel flows through the microopenings to reach the catalytic material.
 11. An ion exchange membranewith an enlarged reactive surface, said membrane is produced by:providing a laser roughened surface; covering the laser roughenedsurface with a solution; allowing the solution to solidify to form anion exchange membrane; and separating the ion exchange membrane from thelaser roughened surface, wherein said ion exchange membrane has anenlarged reactive surface that is a negative replica of the laserroughened surface.
 12. The ion exchange membrane of claim 11, whereinthe solution comprises an electrolyte and a solvent.
 13. The ionexchange membrane of claim 12, wherein the electrolyte is selected froma group consisting of sulfonated ion-conducting aromatic polymer,phosphonated ion-conducting aromatic polymer, carboxylatedion-conducting aromatic polymer and perfluorinated co-polymer, andwherein the solvent is selected from a group consisting of loweraliphatic alcohols, water, and a mixture thereof.
 14. The ion exchangemembrane of claim 11, wherein the enlarged reactive surface is furthercoated with a layer of conductive material.
 15. The ion exchangemembrane of claim 14, wherein the conductive material is a metal or analloy.
 16. The ion exchange membrane of claim 14, wherein the enlargedreactive surface is further coated with a continuous or discontinuouslayer of catalytic material.
 17. The ion exchange membrane of claim 16,wherein the catalytic material is selected from a group consisting ofPt, Pt alloys, V, V alloys, titanium dioxide, iron, nickel, lithium andgold.
 18. An ion exchange membrane with an enlarged reactive surface,said ion exchange membrane is produced by: providing an ion exchangemembrane; providing a laser roughened surface; stamping the ion exchangemembrane with the laser roughened surface; and separating the ionexchange membrane from the laser roughened surface, wherein the stampedion exchange membrane has an enlarged reactive surface that is anegative replica of the laser roughened surface.
 19. An ion exchangemembrane with enlarged reactive surfaces on a front side and a backside, said ion exchange membrane is produced by: providing a mold havingan inner upper surface and an inner lower surface; filling the mold witha solution; allowing the solution to solidify to form an ion exchangemembrane; and separating the ion exchange membrane from the mold,wherein the inner upper surface and inner lower surface of the mold areroughened by laser irradiation, and wherein said ion exchange membranehas an upper surface that is a negative replica of the inner uppersurface of the mold and a lower surface that is a negative replica ofthe inner lower surface of the mold.
 20. A fuel cell assemblycomprising: an anode; a cathode; an electrolyte connecting the anode andthe cathode; and a fuel, wherein said anode comprises an ion exchangesurface enlarged by laser radiation.
 21. The fuel cell assembly of claim20, wherein the ion exchange surface is coated by a layer of conductivematerial and a layer of catalytic material.
 22. The fuel cell assemblyof claim 21, wherein the layer of catalytic material is a discontinuouslayer.
 23. The fuel cell assembly of claim 21, wherein the conductivematerial is a metal or an alloy, and wherein the catalytic material isselected from a group consisting of Pt and Pt alloys.
 24. The fuel cellassembly of claim 20, wherein the electrolyte is a liquid electrolyte.25. The fuel cell assembly of claim 21, wherein the electrolyte is aPEM, and wherein said anode contains micro openings so that the fuel canflow through the micro openings to reach the catalytic material on theion exchange surface.
 26. The fuel cell assembly of claim 20, whereinsaid cathode comprises an ion exchange surface enlarged by laserradiation.
 27. A fuel cell assembly comprising: a fuel; and aPEM-electrode structure comprising a PEM, wherein said PEM is producedby one of: (a) solidifying a solution on a laser roughened surface; (b)solidifying a solution in a mold with a laser roughened inner surface;or (c) stamping an ion-exchange membrane with a laser roughened surface.28. The fuel cell assembly of claim 27, wherein the PEM-electrodestructure further comprise a layer of conductive material and a layer ofcatalytic material.
 29. The fuel cell assembly of claim 28, wherein theconductive material is a metal or an alloy, and wherein the catalyticmaterial is selected from a group consisting of Pt and Pt alloys.
 30. Amethod for producing an ion exchange membrane with a roughened surface,comprising: providing a laser roughened surface; covering the laserroughened surface with a solution; allowing the solution to solidify toform an ion exchange membrane; and separating the ion exchange membranefrom the laser roughened surface, said ion exchange membrane having aroughened surface that is a negative replica of the laser roughenedsurface.
 31. The method of claim 30, wherein the solution comprises aelectrolyte material and a solvent.
 32. The method of claim 31, whereinthe electrolyte is selected from a group consisting of sulfonatedion-conducting aromatic polymer, phosphonated ion-conducting aromaticpolymer, carboxylated ion-conducting aromatic polymer andperfluorinatedco-polymer, and wherein the solvent is selected from a group consistingof lower aliphatic alcohols, water, and a mixture thereof.
 33. A methodfor producing a ion exchange membrane with a roughened surface,comprising: providing a mold having an inner upper surface and an innerlower surface; filling the mold with a solution; allowing the solutionto solidify to form an ion exchange membrane; and separating the ionexchange membrane from the mold, wherein the inner upper surface andinner lower surface of the mold are roughened by laser irradiation, andwherein said ion exchange membrane has an upper surface that is anegative replica of the inner upper surface of the mold and a lowersurface that is a negative replica of the inner lower surface of themold.
 34. The method of claim 33, wherein the solution comprises aelectrolyte material and a solvent.
 35. The method of claim 34, whereinthe electrolyte is selected from a group consisting of sulfonatedion-conducting aromatic polymer, phosphonated ion-conducting aromaticpolymer, carboxylated ion-conducting aromatic polymer andperfluorinatedcopolymer, and wherein the solvent is selected from a group consistingof lower aliphatic alcohols, water, and a mixture thereof.
 36. A methodfor producing a ion exchange membrane with a roughened surface,comprising: providing an ion exchange membrane; providing a laserroughened surface; stamping the ion exchange membrane with the laserroughened surface; and separating the ion exchange membrane from thelaser roughened surface, wherein the stamped ion exchange membrane has aroughened surface that is a negative replica of the laser roughenedsurface.
 37. A method for roughening a surface of an ion exchange systemstructure with laser radiation, comprising: providing an ion exchangesystem structure; providing a source of laser radiation; and irradiatinga surface of the ion exchange system structure with laser radiation tocreate a roughened surface that increases reactive exchange area of saidsurface.
 38. The method of claim 37, further comprising: depositing alaser shading material on the surface of the ion exchange systemstructure, near the surface of the ion exchange system structure, or inthe volume of the ion exchange system structure before laser radiation.39. The method of claim 37, wherein the source of laser radiation is apulse laser.
 40. The method of claim 39, wherein in the pulse laser is aNeYAG laser or an excimer laser.
 41. The method of claim 37, wherein thelaser radiation is provided through a mask to generate shaded areas onthe surface of the ion exchange system structure.
 42. The method ofclaim 37, wherein the laser radiation produces a diffracted image. 43.The method of claim 37, wherein the ion exchange system structure ismade of polymers, ceramics, or silicates.
 44. The method of claim 37,wherein the surface of ion exchange system structure is irradiated withlaser radiation near an ablation threshold of a material beingirradiated.
 45. The method of claim 37, wherein the surface of the ionexchange system structure is irradiated with laser radiation to melt,boil and quench part of the surface.
 46. A method for roughening asurface of an ion exchange system structure with laser radiation,comprising: providing an ion exchange system structure; providing asource of laser radiation; and irradiating a surface of the ion exchangesystem structure with laser radiation to create a roughened surface thatincreases a reactive surface area of said surface, wherein the ionexchange system structure is made of a polymer or a silicate, the sourceof laser radiation is a pulse excimer laser or a NeYAG laser, and theroughened surface is created either by irradiating the surface of theion exchange system structure with laser radiation near an ablationthreshold of a surface material, or by irradiating the surface of theion exchange system structure with laser radiation to melt, boil andquench part of the surface.